Actually, I would argue that "fuck off" is the better response. "I disagree" implies that there's grounds for legitimate discussion when the initial remark is so far out of line there really isn't.
(One of the oddball cases where the "more polite" response may in fact, be more harmful)
[looking the person in the eyes]
"Hey [whoever]"
[repeat as necessary until you have eye contact, at most one or two more times]
"Fuck off."
There is no chance for them to interrupt you mid-protest to make excuses, and it sends a clear signal that the subject is not up for discussion.